This invention relates in general to gas lasers and in particular to the CO2 transverse discharge type with a flowing gas laser medium.
The transverse discharge, transverse gas flow CO2 laser operates in atmospheric pressure laser gas mixtures. The laser makes use of two parallel highly elongated electrodes separated by a small gap within which a short pulse glow discharge is initiated by a high voltage electrical pulse that causes pumping of the upper laser state with subsequent laser emission, the entire pumping and emission sequence taking on the order of several microseconds. Most of the discharge pump power is converted into heating the laser gas which must be maintained at a fixed temperature by a heat exchanger. The two electrodes, the insulating features that hold them in alignment, the high voltage pulser, and the high voltage current feedthrough define a single discharge module.
Each discharge pulse produces ionized gas plasma discharge products within the vicinity of the inter-electrode gap and acoustic shock waves that emanate from the gap area, reflect off surrounding structures, and reenter the gap. Both types of disturbance must be damped or be cleared away before the next pulse, otherwise the subsequent discharge will be disrupted and the quality of the laser output degraded. A background gas flow through the electrode gap performs the plasma clearing function. Acoustic damping, if allowed to proceed by reflection off bare metal surfaces, gives useful discharge repetition frequencies of about 400 Hz. To achieve higher frequencies, acoustic dampers are placed on the vessel side walls, outside the background gas flow so as not to impede it, and the acoustic disturbances are rapidly attenuated by multiple reflection. Acoustic dampers which take the form of wall-mounted perforated metal screens or ceramic material complicate the laser structure, increase its size, and add particulates to the main gas flow in the latter case.
In the case of the CO2 laser, the repetitive glow discharge dissociates CO2 into its stable constituent molecules CO and O2 leading to depletion of the lasing species and a buildup of O2 which degrades discharge uniformity causing highly variable output. Both effects limit the useable lifetime of a sealed gas laser. Lifetime can be extended by replenishing the laser gas from an auxiliary reservoir, but such a reservoir itself has a finite gas supply and it adds considerably to the size and weight of the laser system rendering this approach not practicable in many applications. In order to achieve longer laser lifetime in a compact sealed laser, a CO2 regeneration catalyst is used wherein laser lifetime is extended theoretically indefinitely. Catalysts for CO2 lasers have several forms as reported in the technical literature, including small spheres or cylinders that can be used to fill a catalyst bed, discs that can be placed in a low pressure drop array in a catalyst bed, and single honeycomb structures that can be placed whole in the gas stream. All of these catalyst types have the thin active catalyst material layer loosely bound to an inert substrate, and particles from this layer are continuously liberated to enter the gas flow stream from which they can coat the vessel optical windows and initiate optical damage.
The laser resonator is generally composed of a partially transmitting optic located in the vicinity of one end of the electrodes and another highly reflecting mirror or grating for wavelength selection located at the other end such that the two mirrors or mirror and grating face each other with their optical axes parallel to the electrode long dimension and centered in the electrode gap. Laser output energy scales with the length of the gain medium, length of the electrodes.
The CO2 laser can emit light on approximately 65 different lines, depending upon details of the gain medium and optical resonator designs. Single line emission is of interest in such applications as laser radar and spectroscopy, and individual lines can be selected with a dispersive element such as a grating. However, many of the lines have low gain which often necessitates an increase in gain length, electrode length, to achieve acceptable output, resulting in a significant extension of laser size. This is the case with many of the low gain normal isotope 12C16O2 lines but to a greater extent with other isotopes such as 13C16O2 which emit in important spectral regions where the normal isotope cannot. There is also a requirement for laser beam transverse intensity profile selection between multimode for high statistical diversity and single mode for maintaining the highest intensity in propagating over long distances. Multimode output is favored with short gainlengths (short electrodes) and single mode output is favored with long gainlengths (long electrodes). It would be desirable to have selectability between short and long gainlengths in a single laser to conserve space and eliminate the complexity and cost of multiple lasers.
These problems have received attention in various patents and publications. U.S. Pat. Nos. 4,099,143 and 4,686,680 describe atmospheric pressure, fast gas flow, transverse discharge, transverse gas flow lasers with single sets of discharge electrodes. The gas motive force is supplied by fans that extend the full length of the electrodes and the gas flow describes an essentially circular path through a single set of discharge electrodes, turning vanes, and a heat exchanger. U.S. Pat. No. 4,686,680 shows two heat exchangers within a single flow system with single discharge module.
A study of wall-mounted acoustic dampers is presented in O. Uteza, “Improvement of average laser power and beam divergence of a high pulse repetition frequency excimer laser”, Appl. Phys. B, 66 (1998).
U.S. Pat. No. 5,014,282 teaches how a folded resonator can be employed to take advantage of the large gain volume in a low gas pressure continuous wave laser. U.S. Pat. No. 4,429,398 teaches how a folded resonator can be used to couple two longitudinal gain media for simultaneous excitation and either single or separate dual wavelength output beams.
Various types of catalyst for use in CO2 lasers have been described in Lewis, “Catalyst Selection for a Rep-Pulsed High Power Self-Sustained Discharge CO2 Laser”, SPIE 2702, 385 (1996). The use of catalysts has been described in Willis, “Catalyst Control of the Gas Chemistry of Sealed TEA CO2 Lasers”, J. Appl. Phys. 50 (4) April 1979 and Willis, et al “Use of 13C16O2 in high-power pulsed TEA lasers”, Rev. Sci. Instrum., 50 (9), September (1979). U.S. Pat. No. 4,756,000 teaches how a catalyst can be implemented in a low pressure, longitudinal discharge CO2 laser and also describes a circular flow geometry.
U.S. Pat. No. 5,027,366 teaches how an electrical precipitator can extract particulates from the gas stream of a gas laser and flow the clean gas in a purging curtain over the laser optics that are placed in a purge tube thus opposing the flow of particulates into the tube and onto the optics.